Process for manufacture of micro electromechanical devices having high electrical isolation

ABSTRACT

The present invention relates to a fabrication process relating to a fabrication process for manufacture of micro-electromechanical (MEM) devices such as cantilever supported beams. This fabrication process requires only two lithographic masking steps and offers moveable electromechanical devices with high electrical isolation. A preferred embodiment of the process uses electrically insulating glass substrate as the carrier substrate and single crystal silicon as the MEM component material. The process further includes deposition of an optional layer of insulating material such as silicon dioxide on top of a layer of doped silicon grown on a silicon substrate. The silicon dioxide is epoxy bonded to the glass substrate to create a silicon-silicon dioxide-epoxy-glass structure. The silicon is patterned using anisotropic plasma dry etching techniques. A second patterning then follows to pattern the silicon dioxide layer and an oxygen plasma etch is performed to undercut the epoxy film and to release the silicon MEM component. This two-mask process provides single crystal silicon MEMs with electrically isolated MEM component. Retaining silicon dioxide insulating material in selected areas mechanically supports the MEM component.

This is a division of application Ser. No. 09/075,008, filed May 8,1998, now U.S. Pat. No. 6,159,385.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to micro electromechanical (MEM) systemsand, more particularly, to fabrication of MEM components with a highelectrically isolated substrate.

2. Description of Related Art

Micro electromechanical (MEM) components are being progressivelyintroduced into many electronic circuit applications and a variety ofmicro-sensor applications. Examples of MEM components are radiofrequency (RF) switches, high Q capacitors, pressure transducers andaccelerometers. One such MEM component, a MEM switch, is disclosed inU.S. Pat. No. 5,578,976 which issued to Rockwell InternationalCorporation the assignee of the present application. This MEM switch isfabricated on a GaAs substrate with a cantilevered switch arm formedfrom silicon dioxide deposited upon a sacrificial layer. Contacts andelectrodes are readily formed through deposition of gold and aluminum,respectively.

Another prior art method for creating cantilever beams required a deepanisotrophic etch into a silicon substrate and application of either asilicon nitride or oxide layer to coat the top and side walls of theexposed cut. An isotrophic etch of the silicon substrate undercuts andfrees the MEM component. Unfortunately, this method is not readilyadaptable for applications where non-conductive, high resistancesubstrates such as glass are desired for high isolation applications.

Another method, often referred to as a surface micro machining, uses asacrificial layer such as silicon dioxide deposited on a siliconsubstrate. MEM component material, poly-silicon, by way of example, isthen deposited, patterned and released. The poly-silicon layer is etchedby a reactive ion etch to expose the sacrificial silicon dioxide layer.The sacrificial layer is then etched, usually with an acid (hydrofluoricacid), to release the MEM component. However, MEM components createdfrom poly-silicon have limited mechanical strength and exhibitrelatively poor electrical isolation. Further, production yields arepoor using this method since the wet hydrofluoric etch often results inthe MEM component sticking to the substrate rather than being suspended.

If a high electrical isolation is required, fabrication of MEMcomponents on a glass substrate generally required either ionic(application of high voltage) or fusion (high temperature) bondingtechniques to create MEM components. Both of these bonding techniquesare poorly suited for use when semiconductor devices are present on thesame substrate. Specifically, with ionic bonding the high voltage maydamage sensitive electrical components while the high processingtemperature associated with both ionic and fusion bonding may causejunctions depths to change affecting device performance and reliability.It is also known that such bonding techniques require very smoothsurface to surface contact to ensure a good bond. If the surfaces do notmate within acceptable tolerances, the reaction or inter-diffusionprocess will result in a defective bond. Further, these bondingtechniques are sensitive to surface contamination or irregularitieswhich may result in bond failure sites or a decrease in productionyields.

In another prior art method, a glass substrate is bonded to a silicondioxide layer using ionic or fusion bonding techniques. Prior tobonding, the silicon dioxide layer is deposited on top of a siliconwafer so that the bonding process forms a glass-silicon dioxide-siliconcomposite structure. The silicon is patterned and wet etched to definethe MEM component.

As mentioned above, ionic or fusion bonding require a high processtemperature which are in the range of about 450° C. to 500° C. Further,the glass substrate must be conductive to facilitate bonding with thesilicon dioxide. Such conductivity precludes achieving high electricalisolation in the final MEM system. Further still, with the wet etch usedto release the MEM component, the structure often sticks to thesubstrate rather than remaining free standing.

The present invention provides a method that uses adhesive bonding toform a MEM component on top of a glass substrate so that the MEMcomponent is electrically isolated from the substrates. Further, thepresent process uses a dry etch to release the MEM component. Thus,whatever the merits of the above described prior art methods, they donot achieve the benefits of the present invention.

SUMMARY OF THE INVENTION

The present invention relates to a fabrication process for manufactureof micro electromechanical (MEM) systems having components, such ascantilever supported beams, spaced above the substrate. This fabricationprocess uses as few as two lithographic masking steps depending on thecomplexity of the device and provides MEM components that areelectrically isolated from the substrate.

Specifically, in one embodiment of the present invention, a compositesilicon-film-glass substrate structure is formed. The silicon layer isprocessed by either polishing, grinding or etching to obtain the desiredthickness, patterned to define the MEM component and etched to exposethe film layer. The film layer is a sacrificial layer that is thenpatterned and dry etched to release the MEM component.

In other embodiment of the present invention, the process comprises thesteps of growing a layer of doped silicon on a silicon wafer orsubstrate and depositing a layer of insulating material such as silicondioxide on the doped silicon. This embodiment includes the use of asilicon on insulator (SOI) substrate. The silicon substrate is adhesivebonded to a glass substrate to create a composite silicon-silicondioxide-silicon-adhesive-glass structure. The silicon is patterned andetched using anisotropic plasma dry etching techniques. A secondpatterning then follows to pattern the silicon dioxide layer and anoxygen plasma etch is performed to undercut the adhesive film and torelease the doped silicon MEM component. This two-mask process providessingle crystal silicon MEM component that is electrically isolated fromthe glass substrate but mechanically joined thereto.

The adhesive serves a dual role as a bonding agent and as a sacrificiallayer that can be readily removed to release the MEM component in aneffective and efficient manner. Specifically, the dry oxygen plasma etchundercuts the adhesive without causing the MEM component to stick toadjacent surfaces—a common problem with wet chemical releases. Inaddition the oxygen plasma is a benign process with respect to the othermaterial of the composite structure.

In accordance with the present invention, fabrication of very smallcantilever supported beams, switches or other micro electromechanicalstructures are readily incorporated with other circuit functions on anintegrated circuit device. The present invention is particularly wellsuited for manufacture of long narrow freestanding beams parallel to thesubstrate that can move in response to pressure, electromagnetic,mechanical or other such stimuli.

Also, the surfaces of the substrates need not be perfectly smooth sincethe adhesive layer eliminates the need to use fusion or ionic bondingtechniques to form the composite structure. Indeed, preexistingdiffusions or surface irregularities may be present in the substrateswith little or no impact on yield or the integrity of the compositestructure. This provides very flexible design options since electricalcomponents may be positioned in close proximity to the MEM component.

These and other advantages of the present invention not specificallydescribed above will become clear within the detailed discussion herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectioned view of the first and second substrates which,when combined, will form a composite structure in accordance with thepresent invention.

FIG. 2 is a sectioned view of the substrates, shown in FIG. 1,adhesively bonded to form one embodiment of a composite structure.

FIG. 3 is a sectioned view of the composite structure of FIG. 2illustrating a partial etching to define a micro electromechanical (MEM)component in accordance with the present invention.

FIG. 4 is a sectioned view of the composite structure of FIG. 2illustrating the release of the MEM component in accordance with thepresent invention.

FIG. 5 illustrates, in a general manner, a MEM component manufactured ona glass substrate in accordance with the present invention.

FIG. 6 is a sectioned view of another embodiment of a compositestructure formed in accordance with the present invention.

FIG. 7 shows another alternative embodiment of a composite structure 700from which a MEM component may be readily obtained.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings, which form a part hereof, and inwhich is shown by way of illustration a specific embodiment in which theinvention may be practiced. In the following description, numerousspecific details are set forth in order to provide a throughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-knownstructures and techniques are not shown or discussed in detail in ordernot to unnecessarily obscure the present invention. Reference will nowbe made in detail to the preferred embodiments of the invention,examples of which are illustrated in the accompanying drawings.

Referring now to FIG. 1, process 100 comprises a plurality of stepsperformed on two principal wafer-like elements: a glass substrate 102shown with an organic adhesive layer 104 applied to one surface and asacrificial semiconductor substrate 106. The two substrates are eachindividually processed, combined to form a composite structure and thenfurther processed to define a micro electromechanical (MEM) component.

MEM components are miniaturized freestanding structures spaced above butphysically connected to substrate 102 and electrically coupled to othercircuit elements on substrate 102. Examples of typical MEM componentsmay include cantilever supported switches, diaphragms for pressuresensing applications or suspended beams supported at each end butotherwise physically spaced from the substrate.

Substrate 102 preferably comprises an electrically insulating glassmaterial such quartz, sodium silicate Na₂O—SiO₂ or borosilicateB₂O₃—SiO₂. One preferred substrate uses a high silica glass substratemarketed under the tradename of Vycor, available from CorningIncorporated of Coming, N.Y. Alternatively, in applications where it isdesired to utilize substrate 102 as a conducting element, asemiconductor material may be used. An organic adhesive layer 104 isspun onto substrate 102. Spin coating would provide the most practicalmethod for application of the organic adhesive. However, other coatingmethods such as spray coating, or staged partially-cured thin films,which are applied to the wafer, by way of example, may be used todeposit organic adhesive layer with substantially uniform thickness.

Carrier substrate 106 is either a p-type or n-type silicon wafer such asis commonly used in semiconductor processing; the orientation andconductivity of the wafer will, of course, depend on the specificapplication. Using known semiconductor processing techniques, a siliconlayer 108 is grown on carrier substrate 106. Layer 108 may be doped withboron, germanium or other known dopants to impart an etch stop andsemiconductor properties. Using semiconductor device manufacturingtechniques, electrical components such as resistors, capacitors,inductors or interconnects may be readily formed. An optional silicondioxide layer 110 is grown on top of silicon layer 108 and an organicadhesive layer 112 is spun on top. The silicon dioxide layer may beeliminated if the rigidity of the MEM component is not critical.Further, either adhesive layer 104 or 112 may omitted from the processsince only one layer may be necessary in some applications.

The term organic adhesive refers to thermal setting plastics in which achemical reaction occurs. The chemical reaction increases cross-linkingof the polymer to increase rigidity as well as creating a chemical bondwith the surfaces being mated.

While epoxy is the most versatile type of organic adhesive for thepresent invention, other potential adhesives include polyimide,silicones, acrylics, polyurethanes, polybenzimidazoles andpolyquinoralines. Other types of organic adhesives such as thermalplastics, which require heating above their melting point like wax,would not be of value for this application. The selection of theadhesive would depend in large part on the polymer's thermalcharacteristics and particularly its glass transition temperature. Otherselection criteria include economics, adhesive strength on differentsubstrates, cure shrinkage, environmental compatibility and coefficientof thermal expansion.

The glass transition temperature is the temperature at which chemicalbonds can freely rotate around the central polymer chain. As a result,below the glass transition temperature, the polymer, when cured, is arigid glass-like material. Above the glass transition, however, thepolymer is a softer elastomeric material. Further at the glasstransition temperature, there is a substantial increase in thecoefficient of thermal expansion (CTE). Accordingly, when the glasstransition temperature is exceeded, there is an increase in the CTE andthere is a relief of stress in the polymer layer.

Substrates 102 and 106 may be treated to improve the adhesion of theepoxy. Common treatments include plasma or surface etch treatments. Theuse of a coupling agent or adhesion promoter such as3-glycidoxy-propyl-trimethoxy-silane (available from Dow Corning asZ-6040) or other agents having long hydrocarbon chains to which theepoxy may adhere may be used to improve coating consistency. Wettingagents may be used to improve coating uniformity. However, in mostcases, the coupling agent may serve dual purposes of surface wetting andsurface modification. With the use of organic material, surface finishis not overly critical and the surface need not be smooth. Surfacestructures such as resistors, inductors, capacitors, transistors orconductors may advantageously be added to the surface areas ofsubstrates 102 and 106.

Substrates 102 and 106 are then positioned in a vacuum chamber (notshown) with opposing adhesive layers 104 and 112. The chamber isevacuated to remove air that could be trapped between the substratesduring the mating process. Once a vacuum is achieved, the substrates arealigned and physically joined together to form a composite structurewith a single bead of epoxy. Advantageously, backside alignment marksmay be optically referenced to facilitate the alignment.

Composite structure 200 is shown more clearly in FIG. 2. The organicadhesive layers 104 and 112 combine to form a single adhesive layer 202to bond the composite structure. The adhesive is cured by bakingcomposite structure 200 for a sequence of oven bakes at elevatedtemperatures of up to 180° C. to reduce cure shrinkage. The recommendedcure temperatures depend on the type of epoxy used. Preferably, oncecured, the epoxy is able to withstand elevated temperatures typicallyassociated with many semiconductor processing steps (that is, atemperature up to 250° C.) without additional shrinkage or substantialdegradation.

Further, the adhesive should have a limited amount of cure shrinkage soas to minimize the possibility of inducing cracking or stress fracturesin composite structure 200. The phrase “cure shrinkage” refers to achange in volume of a thermosetting adhesive during the cure cycle dueto the reaction, generally an out-gassing product or rearrangement ofthe polymer itself. Cure shrinkage can cause considerable pressure todevelop along the length of the bond during processing.

An additional component closely associated with cure shrinkage is theadhesive's CTE at the cure temperature and at the operating temperature.Generally, organic adhesives will have a higher CTE than any othermaterial present in the MEM component. This means that the adhesive willexpand at a higher rate than will other material as the ambienttemperature increases. Although an inorganic filler material may beadded to the adhesive to reduce the CTE such fillers are not recommendedin this application since such inorganic filler material cannot beetched using oxygen plasma during the structural releasing step.

Substrate 102 may also be selected for a desired CTE. The substrate inone preferred embodiment has a CTE that is substantially similar to butless than that of silicon so that the resulting MEM component will beslightly stretched (or in tension). In this manner, smaller structureswill retain some degree of elasticity and will be less susceptible todeveloping a bow or collapsing upon release of the MEM component. Itwill be appreciated that if the CTE of substrate 102 is greater thanthat of silicon, the MEM component will be in compression upon release.Larger beams may be capable of retaining the structural integrity incompression after all process steps are complete. Use of glass substrate102 was selected in the described embodiment primarily to achieve highelectrical isolation. However, one skilled in the art will appreciatethat the process steps of the present invention may be applied toselected substrates other than the above described glass substrate duethe versatility of the bonding and sacrificial layer.

Due to the number of material types present in the composite structure200, thermal processing and the mismatching of thermal coefficients mayimpart shear fractures or stress cracks in one or both substrates or inthe various layers. The CTE in the bond line of the MEM component willdepend upon the polymer used, its curing agent, and the thermal cycleused to cure the polymer. When the processing and operating temperaturesare maintained so that the polymer is not exposed to temperaturesexceeding the glass transition temperature, the polymer is a rigidglass-like material and the CTE is minimized. If the glass transitiontemperature is exceeded, however, there will be an increase in the CTE,which may result in a relief of stress in the polymer layer. Any suchstress relief after composite structure 200 is formed may deformcomposite structure 200. Since flexible or elastomeric polymers willhave a high CTE, a thin bond line is desirable to minimize structuraldamage in the event the glass transition temperature is inadvertentlyexceeded. In one preferred embodiment, the bond line has a thickness ofabout five (5) to seven (7) microns.

Referring again to FIG. 1, the next step is to remove substrate 106 andexpose silicon layer 108. Substrate 106 is removed using a backsidechemical etch. A mechanical grinding or polishing step may proceed thechemical etch to reduce the amount of silicon etching required or toreduce the selectivity needed in the etch stop. Since substrate 106primarily functions as a sacrificial carrier for silicon layer 108,there is no need to preserve substrate 106 in the region of the MEMcomponent so as to facilitate subsequent processing of silicon layer108. Alternatively, if a specific application required thick or rigidbeams, one skilled in the art will appreciate that substrate 106 couldbe patterned and selectively etched for use as beam material or retainedin adjacent areas as a substrate for integrated circuit devices.

As shown in FIG. 3, an optional layer of aluminum 116 is deposited ontothe exposed surface of silicon layer 108. Aluminum 116 is patterned witha first mask to form conducting areas and to expose selected portions ofsilicon layer 108. The aluminum is either lifted off of etched using analuminum-specific chemical agent. An oxide or nitride cap 115 may beapplied on top of the aluminum layer 116 to form a protective barrier.

With silicon layer 108 exposed through the aluminum 116, an anisotropicetch defines the structural dimensions of the MEM component. This etchstops upon reaching the silicon dioxide layer 110.

A second mask is applied to pattern the insulating silicon dioxide layer110 and then etched. It is important to note that due to its insulatingproperties, portions of silicon dioxide layer 110 may be retained tomechanically support selective areas of the MEM component whilemaintaining electrical isolation. Specifically as shown in FIG. 5, holes150 may be etched into silicon dioxide layer 110 to facilitate etchingadhesive layer 120.

Referring now to FIG. 4, adhesive layer 120 is etched in a final processstep to release the MEM component. In the preferred embodiment, dryoxygen (O₂) plasma etch undercuts adhesive layer 120. Since this plasmaetch is a dry release, manufacturing yields are improved relative to wetetching. Specifically, as one familiar with semiconductor technologywill appreciate, wet etching may lower production yields as watermolecules get trapped under the released MEM components causing the MEMcomponent to stick to substrate 102.

Upon MEM component release, stress forces along the bond line will besubstantially released. Further, any pressure generated by shrinkage ofthe organic adhesive during cure will be significantly reduced afterprocessing by dicing the wafers into discrete devices resulting inmechanically stable devices.

Another embodiment of a composite structure formed in accordance withthe present invention is shown in FIG. 6. Structure 600 comprises afirst and second substrate 106 and 102. Specifically, substrate 102 ispreferably a glass substrate that exhibits high electrical isolationwhile substrate 106 is a semiconductor substrate. Substrate 106 may beeither p or n-type and may be selectively doped to impart conductive orsemiconductor properties. Substrates 102 and 106 are joined together byan adhesive film 602.

If preferred, substrate 106 may be ground and polished to a desiredthickness, patterned to define a MEM component and etched to expose theadhesive film 602. The film is then patterned and dry etched to releasethe MEM component. As will be understood, substrates 102 and 106 mayfurther include conductive elements such as switch contacts orelectrodes (not shown).

FIG. 7 shows another alternative embodiment of a composite structure 700from which a MEM component may be readily obtained. Silicon on insulator(SOI) substrate 702 has a buried layer of silicon dioxide layer 704 thatacts as an etch stop. The buried silicon dioxide layer can be formed byseveral commercially available techniques, including implanting oxygen(O₂), deep into substrate 702. The silicon-oxide-silicon (which may beeither doped to impart semiconductor properties or undoped) isadhesively bonded to substrate 706 to form a composite structure. In thepreferred embodiment, substrate 706 is a glass substrate. Using thetechniques described above, the MEM component is defined and released bythe two-mask patterning and etching process.

Referring again to FIG. 5, a topological view of an integrated circuitcomprising a MEM component is shown. This circuit is an exemplary viewof one type of MEM component that may be created using the process ofthe present invention. Beams 152 of about one micrometer wide andseveral millimeters in length are constructed using the above-describedprocess. The thickness of these beams may range from about 20-30micrometers up to about 100 micrometer. One skilled in the art willappreciate that the above-described dimensions may be readily changeddepending on the specific application. Support beams 154 may addrigidity to selected ones of beams 152.

With the present invention a very high aspect ratio may be achieved forthe MEM component. By way of example, very narrow, deep MEM sensors areobtained using the above described process steps. Thus this type ofsensor will have high capacitance value that is easy to detect.Advantageously, sensor interface may be achieved by electrically bondingto pads 156.

The process of the present invention is independent of the substratematerial. Since epoxy or organic adhesives, in general, readily bond awide variety of substrates, substrates may be selected depending on thespecific application in which the MEM component will interface. The lowtemperature bonding readily enables the use of gallium arsenide, siliconor other material on a glass substrate. Further, the process is readilyapplied to applications where silicon or gallium arsenide substrates aredesired instead of a glass substrate.

It will be appreciated that it is also very desirable in manyapplications to position electronic circuits in close proximity to theMEM component. With the present invention, transistors or other circuitelements may be fabricated directly on the silicon, prior to creatingthe composite structure of FIG. 2. Further, with the present invention,structures can be formed on both sides of the beam. By way of example,aluminum can be deposited between the silicon dioxide and the siliconlayers (see FIG. 1) to create a very conductive structure. One skilledin the art will appreciate that the low process temperatures employed tocreate the composite structure, to define or to release the MEMcomponents has minimal impact on junction depths or circuit deviceparameters.

While certain exemplary preferred embodiments have been described andshown in the accompanying drawings, it is to be understood that suchembodiments are merely illustrative of and not restrictive on the broadinvention. Further, it is to be understood that this invention shall notbe limited to the specific construction and arrangements shown anddescribed since various modifications or changes may occur to those ofordinary skill in the art without departing from the spirit and scope ofthe invention as claimed.

We claim:
 1. A microelectromechanical (MEM) device comprising: a) afirst substrate defining an upper surface that extends along a planedefined by a lateral direction and a longitudinal direction; b) a secondsubstrate including: i. first and second longitudinal outer membersdisposed at opposing longitudinal outer ends of the second substrate andpermanently connected to the upper surface of the first substrate; andii. a central member connected between the first and second longitudinalouter members, wherein the central member has first and second lateraledges that define a thickness of the central member, the central memberdefining a void disposed between at least a portion of the centralmember and the upper surface of the first substrate, wherein the void iscontinuous between the first and second lateral outer edges of thecentral member.
 2. The MEM device as recited in claim 1, wherein theportion of the central member is suspended above the first substrate. 3.The MEM device as recited in claim 1, wherein the central member isintegrally connected to the first and second longitudinal outer members.4. The MEM device as recited in claim 1, wherein the second substrate iselongated along a direction defined by the longitudinal axis.
 5. The MEMdevice as recited in claim 1, wherein the longitudinal outer members areconnected to the first substrate via an adhesive.
 6. The MEM device asrecited in claim 1, further comprising a conductive layer disposed ontop of the central member.
 7. The MEM device as recited in claim 6,wherein the conductive layer comprises aluminum.
 8. The MEM device asrecited in claim 6, further comprising a protective barrier disposed ontop of the conductive member.
 9. The MEM device as recited in claim 8,wherein the protective barrier is selected from the group: consisting ofan oxide and a nitride.
 10. The MEM device as recited in claim 1,wherein the first substrate is insulating.
 11. The MEM device as recitedin claim 1, wherein the second substrate further comprises a layer ofsilicon having an insulating layer formed thereon.
 12. The MEM device asrecited in claim 11, wherein the insulating layer comprises silicondioxide.
 13. The MEM device as recited in claim 11, wherein the centralmember is formed by attaching the second substrate to the firstsubstrate via an adhesive, and removing at lease a portion of theadhesive.
 14. The MEM device as recited in claim 1, wherein the secondsubstrate comprises an SOI wafer.
 15. The MEM device as recited in claim1, wherein the thickness of the central member is substantially onemicrometer.
 16. The MEM device as recited in claim 1, wherein thethickness of the central member is between 20 micrometers and 100micrometers.
 17. The MEM device as recited in claim 1, wherein thecentral member has a length greater than 1 millimeter.